В чем ошибка, C++

[123Ллла]

#include <fstream>
using namespace std;
const char CDfv [] = "Duomenys3.txt";
const char CRfv [] = "Rezultatai3.txt";
int S(int a, int b);
int main ()
{
    int x,y,s;
    ifstream fd (CDfv);
    ofstream fr (CRfv);
    fd >> x >> y;
    s= S(x,y);
    fr << s;
    fd.close ();
    fr.close ();
    return 0;
}
int S(int a, int b);
{
    return (a*b)/2;
}

|19|error: expected unqualified-id before '{' token|

[BDA]

int S(int a, int b)
{
    return (a * b) / 2;
}

";" лишняя.